To use Siri with Microsoft To-Do, you can connect your Outlook.com or Exchange account. Just head to your iPhone's settings, select Accounts & Passwords and then Add Account. Then select Exchange or Outlook.com and simply follow the prompts to add your account.
Once you’ve added your account, you’ll also need to enable Reminders for that account. Just select Accounts & Passwords once more and then select either Exchange or Outlook.com. Then make sure that Reminders is toggled on for that account.
Once your Microsoft Account has been connected, you'll need to set your default Reminders list to one in your Outlook or Exchange account. Just open your iPhone's settings, tap Reminders > Default List and then select the desired list.
Now you can simply say 'Hey Siri, remind me to...' and it will be synced to Microsoft To-Do. Please know, however, that Siri does not currently work with business customers who are using InTune.